Delta has started to require more bags be checked. They used to check a quite a few at teh gate for free. Now they are making you go back to the ticket counter and pay the fee.
I fly delta a LOT. They started out to be pretty lax about checking bags plane side. I've seen it get worse and worse to the point of abuse as huge bags are drug to the gate as “carry on”.
They may have no choice but to crack down on the abuse.
Other carriers such as British Airways will make you put your bag into the little “you bag must fit here” thingie before you can carry it on.